With the development of power electronics, servo motors have become widely used in many applications. The servo motor, as a special type of motor, has a working principle in electrical signals are transformed into angular displacement or angular velocity of the output shaft. The servo motor is easy to control, has a small volume, is relatively light weight with high output power and torque.
FIG. 7 illustrates a rotor structure of a conventional servo motor, and FIG. 8 is a graph of cogging torque verses the mechanical angle of the rotor of a conventional servo motor. As shown in FIG. 8, the torque of the servo motor of the conventional art varies in a comparatively large amplitude as the mechanical angle of the rotor varies. In this example, the difference value between the maximum value and the minimum value is about 177 mNm. This variation in the torque is known as cogging torque. Cogging torque produces cogging or a tendency for the rotor to snap to preferred angular positions. The greater the cogging torque, the greater this tendency to snap. Cogging is particularly noticeable in low speed operation as the rotor tries to snap or jump from one prepared position to the next. It also affects positional control as the rotor becomes unstable between the preferred angular positions. Generally speaking, in a permanent magnet motor, cogging torque usually is the basic cause of vibration and noise and adversely affects control precision. In a variable speed drive, when the torque frequency is consistent with the mechanical resonance frequency of the stator or rotor, the vibration and noise produced by the cogging torque will be amplified. Cogging torque also affects the low speed performance of the motor in a speed control system and the accurate positioning of the motor in a position control system.
Thus there is a desire for a servo motor which has reduced cogging torque.